Interplay of $d$- and $p$-states in RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ flat-band kagome metals

Abstract

Shifting the Fermi level of the celebrated $AM_3X_5$ (135) compounds into proximity of flat bands strongly enhances electronic correlations and severely affects the formation of density waves and superconductivity. Our broadband infrared spectroscopy measurements of RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ combined with density-functional band-structure calculations reveal that the correlated Ti $d$-states are intricately coupled with the Bi $p$-states that form a tilted Dirac crossing. Electron-phonon coupling manifests itself in the strong damping of itinerant carriers and in the anomalous shape of the phonon line in RbTi$_3$Bi$_5$. An anomaly in these spectral features around 150 K can be paralleled to the onset of nematicity detected by low-temperature probes. Our findings show that the materials with low band filling open unexplored directions in the physics of kagome metals and involve electronic states of different nature strongly coupled with lattice dynamics.